BEM

OOCSS: concepts help us write components that are flexible, modular and interchangeable.

Atomic CSS: is the approach to CSS architecture that favors small, single-purpose classes with names based on visual function.

SMACSS: I’ve been analyzing my process (and the process of those around me) and figuring out how best to structure code for projects on a larger scale. What I've found is a process that works equally well for sites small and large.